Internally Cross-Linked Polysulfone-Based Anion Exchange Membranes with pH Stability for Carboxylate Separation

Carboxylates, which are considered high-value platform chemicals, are by-products produced during the fermentation of organic waste. There is a major challenge in effectively separating and recovering carboxylates from the fermentation broth. Electrodialysis (ED) has been considered a promising technology with its high selectivity in the absence of chemical additives. Here, a class of internally cross-linked anion exchange membranes (referred to as QPSF-n AEMs) were synthesized by using chloromethylated polysulfone (CMPSF) and varying amounts of 1,4-diazabicyclo [2.2.2] octane through quaternization and cross-linking reactions to recover carboxylates. The resulting membrane (QPSF-0.88) showed a high anion exchange capacity of 2.79 mmol g–1 but a low water content of 3.3% and an excellent swelling rate of 13.9%. To evaluate the separation efficiency of carboxylates, the membrane was tested with single solutions of acetate, oxalate, and citrate in 15.0 V (constant voltage) ED for 180 min. The removal efficiencies of QPSF-0.88 for acetate, oxalate, and citrate were 92.6, 87.9, and 85.9%, respectively, indicating a monocarboxylate selectivity for QPSF-0.88. After immersion in acidic or basic solutions, the change in the ion exchange capacity of QPSF-0.88 is less than 2%, indicating a high resistance to pH changes. This study presents innovative concepts in developing a membrane for the desalination of carboxylate.